(1) Field of the Invention
This invention relates to a tire for use in running on a soft ground, and more particularly, to an improvement on a tread profile having tread patterns composed of a group of blocks, which is applied to a motocross tire of a relatively large scale motorcycle.
(2) Description of the Prior Art
A tire running on the soft ground, for instance, a motocross tire, has conventionally been constituted such that the groove between the adjacent blocks is made wider, and the net ground-contacting area to the total tread area including the grooves is set as extremely low as 15-30% from the requirements for the mud throw-out property, the traction performance, etc.
Recently, in order to further enhance the penetrating force of the blocks into the relatively soft ground surface and improve the controllability of the traction, etc., there is a tendency for the blocks to be designed in a smaller size and a hard tread rubber is used for preventing the reduction in the rigidity of the blocks which may be caused thereby.
However, if the hardness of the tread rubber is increased, the required items are met, but on the other hand, rubber flow is difficult during the vulcanization step, and the rubber near the blocks, that is, the rubber layer under the inner surfaces of the blocks is sucked into block forming portions of the mold, so that the carcass layer at these portions is drawn to form a wave shape and accordingly the durability of a casing is decreased. Further, since the fluidity of the rubber at a portion between the blocks, that is, at the portion corresponding to the groove portions drops, the gauge of a skid base at the groove portions becomes conspicuously thicker for the gauge at the block positions. Consequently, the rigidity of the whole tread portion is increased, and particularly, the thickness of the groove bottom portions becomes increased. Thus, the flexural rigidity in running under load becomes higher and the ground-contacting area becomes smaller. Thereby, not only steering performance such as the traction controllability, etc. are deteriorated, but also the weight of the whole tire is unfavorably increased, while the tire-manufacturing cost is raised.